You also have to watch the guppies. Guppies are known to nip betta fins and can stress your betta. Some people have it work though. It all really depends on your betta. The one thing to remember with an aggressive species like a betta is to always monitor them carefully. They might be ok with the situation one day and then decide they don't like the situation anymore the next day. This can either turn into a bloodbath where the betta kills the other fish or the betta slowly wastes away from the stress of living with tank mates he/she doesn't like. Just keep on top of watching your fish and you should be ok. Good luck and keep us updated.