I wouldn't personally add guppys to your tank, apart from the angels nipping, i have a RTBS myself and i have noticed that if you have red tailed guppys, or any guppys with a distinctive brightly colored tail, it can go for them every now and then- given that my rtbs has calmed down a huge amount since the couple of months i have had him, guppys can be very prone to getting stressed and then sick and so i wouldn't risk having them in your tank for the simple reason you may encounter alot more fish deseases if the guppys feel uncomfortable with so many large fish around.
Guppys can be very rewarding fish, although they are often considered "beginner fish", they can actually be quite difficult to keep from time ti time. I think your best bet if you still want to keep guppys, would be to get another tank- it can be anything from 10-40gals+ if you like, although 10gallons is the advised minimum, in a guppy only 10gal tank you could happily fit 6 guppys
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Guppy keeping is quite basic- in a mixed gender group, you need at least 2-3females per male, in an all-male group its advised you have at least 5males to help disperse any agression, and in all-female groups it doesn't really matter how many females you have as long as you have 3 as guppys are very sociable fish
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For feeding guppys, its advised that you give them a mix of vegetable and high protein based foods every week- although they can live entirely off fish flakes, fish flakes aern't that healthy for fish and can lead to constipation. For veg, things like cumcumber, melon, sliced tomato, algae wafers and (cooked)peas will all be readily accepted, for hig protein foods you can feed them things like krill, daphinia, tubifex, bloodworms, muscles, cockels, squid and prawns
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Female guppys can store sperm in them, so an all-female group won't nesarsarily prevent you from having fry and guppys are notorious for their ability to regually breed- a single female can produce up to 40+fry a month. Although adult guppys can eat guppy fry from time to time, you need to have a plan on what you will do if you they don't eat the fry. If you don't want fry, your angels will eat up the fry in a second as long as the fry are preferably not older than 2weeks old, if you want to raise some fry, a separate fry tank of at least 5gals is advised- theres alot of info on livebears and raising fry in the pinned articles here for more info
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