Is there any way to make ISO settings independent based on exposure mode selected, meaning we could set ISO 200 for M mode and ISO 400 in Av mode and when the dial is switched from M to Av, the ISO will be 400, not 200?
Since the entry level Rebel T6/1300D, along with the entire Rebel/xx0D/XX00D series, has no 'C' position on the mode dial this is not a feature available with that model. The 77D, which is the successor to the Rebel T6S/760D, is also like the Rebel series and offers no 'Custom Mode' on the mode dial.
EOS Rebel T6/1300D mode dial
On Canon bodies with user designatable "Custom Modes" ('C1', 'C2', etc. on the mode dial¹) this is possible. The user puts the camera in whatever state desired for pretty much all of the settings available through either control buttons or a myriad of menu options, then "records" the current state of the camera and all of those settings to one of the 'Custom' modes.
The user can then alter whatever settings are desired and shoot with the camera using whatever options they so choose. When the mode dial is later turned to the 'Custom Mode' used to previously record the state of the camera, the camera and pretty much all of its settings will be returned to the same state they were in when the user recorded that 'Custom Mode'.
Not only are things such as ISO, Av, Tv, Exposure Compensation, etc. recorded to one of the 'Custom Modes', but almost all menu items such as metering mode, AF mode, AF point (or area), bracketing, color temperature and white balance correction, etc. are also recorded when the current state of the camera is saved as a 'Custom Mode'.
Some models have a single 'Custom Mode' available for recording the state of the camera (70D, 60D), others have two (80D, 50D, 6D, 6DII, M5) or three 'Custom Modes' available (1D X II, 1D X, 1D IV, 1Ds III, 5D IV, 5Ds/5Ds R, 5D III, 5D II, 7D II, 7D). In general, the models with more automatic modes ('Scene' modes and/or more than one 'Full Auto' mode) available directly on the mode dial lack the 'Custom Mode' feature. Models with fewer or only a single 'Auto' mode and no individual 'Scene' modes on the dial tend to offer more user recordable 'Custom Modes'.
The mode dial of the Canon EOS 5D Mark III
EOS 5D Mark III mode dial
The mode dials of the EOS 7D Mark II, 5Ds, 5Ds R, and 5D Mark IV are identical. The dials on the EOS 7D and 5D Mark II lack the center 'locking button' that must be depressed to move the dial, but are otherwise similar.
The EOS 6D and 6D Mark II include two 'Custom Modes' on the mode dial, 'C1' and 'C2', along with two "Auto" modes - 'Scene Intelligent/Auto+' and 'Creative Auto' - and a 'Scene' mode position that allows access to multiple 'Scene' modes using the control wheels. The 80D, 50D, and M5 also have two 'Custom Modes' available.
The EOS 80D includes the following on its mode dial: Scene Intelligent/Auto+ (Stills and Movie), No Flash, Creative Auto, SCN(Portrait, Landscape, Close-up, Sports, Night Portrait, Handheld Night Scene, HDR Backlight Control, Food, Kids, Candlelight), Creative filters, Program AE, Tv, Av, Manual (Stills and Movie), Bulb, 'C1', and 'C2'.
EOS 80D mode dial faceplate
The 70D is similar, but only offers a single 'C' mode and no 'Creative Filters' mode. The selections available under the 'Scene Mode' position are also slightly different.
¹ The top of the line 1-Series from Canon has no mode dial. The modes are accessed by holding down the 'Mode" button and turning either the 'Main Control Dial' (directly behind the shutter button) or the 'Quick Control Dial' (the large wheel on the right side of the back of the camera) to scroll through the list of 'Exposure Modes' available. The most recent 1-Series Canon EOS camera, the 1D X Mark II, allows three different 'Custom Modes' to be recorded and recalled.
This is the list of settings that are recorded by the EOS 5D Mark III when one of the three 'Custom Modes' is registered. Other EOS cameras with 'Custom Setting' capability are similar. The current state of each of these settings is recorded for that 'Custom Setting' (e.g. 'C1', 'C2', or 'C3'). From pages 334-335 of the EOS 5D Mark III Instruction Manual²:
² Different editions for various firmware revisions of the EOS 5D Mark III
Instruction Manual may have slightly different page numbers. For instance, in the manual covering firmware version 1.2.0 and later, the same information is on pages 339-340.