ProMeth

For compatibility reasons, the 8 pin EPS connector can split into two 4 pin EPS connectors, like this:

 
From the PSU manual (see B):

Rx 550 is better than Vega 11

Should work fine yes. 

it should work just fine..just that msi only did a test on a single stick sniper x model..

https://www.msi.com/Motherboard/support/B450-TOMAHAWK-MAX#support-mem-19

I'm guessing you meant MB520?
The H45 would probably be the best performer. https://www.vedantcomputers.com/products/cpu-cooler/corsair-hydro-series-h45
Check with the seller to make sure it's updated with the AM4 mounting hardware or you can get it from Corsair. 
If they don't, there's another listing on Amazon from Corsair that's likely to have updated mounted hardware: https://www.amazon.in/Corsair-Cooling-Hydro-H45-Black/dp/B01FSMWFQG/ref=sr_1_3?keywords=corsair+h45&qid=1571755585&s=computers&sr=1-3#customerReviews

From my experiences, 3600 does x264 Fast 1080p60 just fine with heavy-ish games.

The R5 3600 and then use NVENC, your gaming performance will be higher at small cost of stream quality which shouldn't matter unless you literally live from streaming and need the best quality possible.