Is Full-size speakers for L-R Rears advisable?

I’m slowly upgrading my Stereo setup to HT.

Currently, I’ve full-size (FS) Tyler Acoustics Linbrook System II speakers for LR connected to Luxman M-900u Amplifier (150W @8 Ohm) & matching Center speaker (unused). To match sonically, I’m considering buying a pair of either bookshelf or similar/ better FS speakers from the same brand (Tyler Acoustic).

The questions I have are:

1.) Will a pair of FS rear eliminate the need for a dedicated Subwoofer or it would be better to get a dedicated Subwoofer & buy either a pair of bookshelf or FS speakers for Rears?

2.) If I get a dedicated Subwoofer, getting a FS speakers for rear will make any difference compared to Bookshelf speakers?
3.) Will I notice any positive difference if I buy more expensive fullsize pairs from Tyler & use them as Front LR & move the existing Linbrook System II pairs to Rear or it’s always better to keep the LRC with the same model/ series?
4.) Since they’ll be connected to a separate Amp anyways, does it matter if the Front & Rear speakers have different impedance (8 vs. 4 Ohms)?
5.) How should I drive the remaining 3 channels? Should I get 2 pairs of Stereo Amps (one will remain unused) or buy 3 Mono Amps?
6.) Does Amplifier’s Power Output wattage also have to exactly match for all the 5 channels or it can be close (+/- 50 Watts)?

7.) Do the Amps also have to be from the same brand? 

LRC: Linbrook System II
Frequency Response: 32-25k
Sensitivity: 89db
Impedance: 8 ohm
Power: 30-250wpc