100bi can be a usual for sng, for large field mtt's many use a 200bi min br. I personaly plan to maintain a 50bi for cash if i can manage to get my roll moving in the right direction, maybe take a shot at 40bi just to be nitty on the br with it all being a challenge of brm and grinding only in the last couple of months.
I'm sure you could use maybe 20bi cash, 50bi sng, 100bi mtt rules, but do believe these are abs min brm rules to look at. Even in a standard sng varience can strike to wind up losing 5 or 10 in a row easily so just my thoughts to have a bigger br cushion for them.
If you are mixing up both at the same time and grinding multiple, I would go with the higher requirement accross the board. Just a thought that if you are playing 5nl because your rolled for it but only 1.5 sng's because those are all your rolled for in sng bi rules a 1 bi loss on the cash table just busted 3 sng bi's, and could see this easily causing tilt if not well managed.
Just my thoughts, but then again I plan to be a br nit when I finally get there with a proper roll. Right now I'm just playing to lowest of 2nl (on ftp, 38$ roll) and 1.5sng's (fifty50's to be exact on stars as had 10$, played a little cash didn't go well and have build from 6$ now up to 25$ through the Fifty50 and a scatter reg sng). It's currently working so continuing for now and hoping to get a more comfy role everywhere haha.
You mention if standardly play cash and looking to work sng's into your game, then you could always set aside say 5 or 10bi stop loss in the sng's and see how it goes. If you lose it build it back with cash and try again, rince and repeat until you have improved to build a br through sng's or obv vice versa. Shot taking (within reason obv) never hurts, as long as you don't let taking a shot be a huge impact to a br or set you on tilt in any way.