Pyqt5 Qstackedwidget импорт своего класса

Если вы редактируете через QtDesigner, то описал ниже, если программно, то VideoStream можно сразу добавить в Ui_Form.

Добавьте в self.page лейаут, например QVBoxLayout, он нужен чтобы растянуть его содержимое на весь родительский виджет:

lass Ui_Form(object):
    def setupUi(self, Form):
        Form.setObjectName("Form")
        ...
        self.page = QtWidgets.QWidget()
        layout = QVBoxLayout()
        self.page.setLayout(layout)        
        ...

Далее в main.py

from video import VideoStream

class MainClass(QtWidgets.QMainWindow, Ui_Form):
    def __init__(self):
        super().__init__()
        self.setupUi(self)

        # Добавим VideoStream в лейаут на self.page
        self.page.layout().addWidget(VideoStream())

Я отметит для вас места где внес изменения.

У меня все модули в одном каталоге. Когда проверите мой пример, разместите свои модули как вам будет угодно.

Чтобы разместить что-то в label_2, создайте layout наследованный от label_2 и размещайте туда что хотите.

main.py

import sys
from PyQt5 import QtWidgets
from PyQt5 import QtCore
#from simple.gui import Ui_Form
from gui import Ui_Form                                       # +

from video import VideoStream                                 # +++

class MainClass(QtWidgets.QMainWindow, Ui_Form):
    def __init__(self):
        super().__init__()

        self.setupUi(self)
        self.pushButton_2.clicked.connect(lambda : self.stackedWidget.setCurrentIndex(1))
        self.pushButton.clicked.connect(lambda : self.stackedWidget.setCurrentIndex(0))

# +++ vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv
        layout = QtWidgets.QVBoxLayout(self.label_2)
        self.videoStream = VideoStream(self)
        layout.addWidget(self.videoStream)

        self.pushButton_3.clicked.connect(self.videoStream.controlTimer)
# +++ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

if __name__ == "__main__":
    app = QtWidgets.QApplication(sys.argv)
    win = MainClass()
    win.show()
    sys.exit(app.exec_())

video.py

import cv2
from PyQt5.QtCore import QTimer
from PyQt5.QtGui import QImage
from PyQt5.QtGui import QPixmap
from PyQt5.QtWidgets import QWidget
#from simple.gui import Ui_Form


class VideoStream(QWidget):                                # - , Ui_Form):
    def __init__(self, parent=None):                       # + parent
        super().__init__(parent)                           # + parent
        self.parent = parent                               # + parent

# ---       self.setupUi(self)                                                # ----

        # create a timer
        self.timer = QTimer()
        # set timer timeout callback function
        self.timer.timeout.connect(self.viewCam)
        # set control_bt callback clicked  function
#        self.pushButton_3.clicked.connect(self.controlTimer)                  # ----

    # view camera
    def viewCam(self):
        # read image in BGR format
        ret, image = self.cap.read()
        # convert image to RGB format
        image = cv2.cvtColor(image, cv2.COLOR_BGR2RGB)
        # get image infos
        height, width, channel = image.shape
        step = channel * width
        # create QImage from image
        qImg = QImage(image.data, width, height, step, QImage.Format_RGB888)
        # show image in img_label
        self.parent.label_2.setPixmap(QPixmap.fromImage(qImg))                   # self.parent.label_2

    # start/stop timer
    def controlTimer(self):
        # if timer is stopped
        if not self.timer.isActive():
            # create video capture
            self.cap = cv2.VideoCapture(0)
            # start timer
            self.timer.start(20)

        # if timer is started
        else:
            # stop timer
            self.timer.stop()
            self.cap.release()

gui.py

from PyQt5 import QtCore, QtGui, QtWidgets

class Ui_Form(object):
    def setupUi(self, Form):
        Form.setObjectName("Form")
        Form.resize(534, 325)
        Form.setStyleSheet("background-color: rgb(1, 203, 170);")
        self.stackedWidget = QtWidgets.QStackedWidget(Form)
        self.stackedWidget.setGeometry(QtCore.QRect(20, 70, 491, 241))
        self.stackedWidget.setObjectName("stackedWidget")
        self.page = QtWidgets.QWidget()
        self.page.setObjectName("page")
        self.label = QtWidgets.QLabel(self.page)
        self.label.setGeometry(QtCore.QRect(80, 70, 311, 101))
        font = QtGui.QFont()
        font.setPointSize(25)
        self.label.setFont(font)
        self.label.setObjectName("label")
        self.stackedWidget.addWidget(self.page)
        self.page_2 = QtWidgets.QWidget()
        self.page_2.setObjectName("page_2")

        self.label_2 = QtWidgets.QLabel(self.page_2)
        self.label_2.setGeometry(QtCore.QRect(30, 30, 331, 191))
        font = QtGui.QFont()
        font.setPointSize(30)
        self.label_2.setFont(font)
        self.label_2.setObjectName("label_2")


        self.pushButton_3 = QtWidgets.QPushButton(self.page_2)
        self.pushButton_3.setGeometry(QtCore.QRect(380, 110, 75, 23))
        self.pushButton_3.setStyleSheet("background-color: rgb(255, 85, 255);")
        self.pushButton_3.setObjectName("pushButton_3")
        self.stackedWidget.addWidget(self.page_2)
        self.pushButton = QtWidgets.QPushButton(Form)
        self.pushButton.setGeometry(QtCore.QRect(70, 20, 75, 23))
        self.pushButton.setStyleSheet("background-color: rgb(0, 85, 255);")
        self.pushButton.setObjectName("pushButton")
        self.pushButton_2 = QtWidgets.QPushButton(Form)
        self.pushButton_2.setGeometry(QtCore.QRect(400, 20, 75, 23))
        self.pushButton_2.setStyleSheet("background-color: rgb(255, 85, 0);")
        self.pushButton_2.setObjectName("pushButton_2")

        self.retranslateUi(Form)
        self.stackedWidget.setCurrentIndex(0)
        QtCore.QMetaObject.connectSlotsByName(Form)

    def retranslateUi(self, Form):
        _translate = QtCore.QCoreApplication.translate
        Form.setWindowTitle(_translate("Form", "Form"))
        self.label.setText(_translate("Form", "Simple first page"))
        self.label_2.setText(_translate("Form", "Video on opencv"))
        self.pushButton_3.setText(_translate("Form", "start"))
        self.pushButton.setText(_translate("Form", "Main"))
        self.pushButton_2.setText(_translate("Form", "Video"))