Bristlenose pleco will reach the size of 4~5" long. Even a 10 gallon tank is not recommended for them.
Betta fish will not get lonely. In their natural habitat, their are loners.
You are right, it is not recommended to have middle level swimmers in the same tank as betta. Either betta will be aggressive to them, or they will nip at betta's long fin.
If you absolutely have to get a tank mate for your betta, I recommended zebra nerite snails. No more than 2~3 in a 10 gallon tank along side of the betta fish.
For now, you should focus on getting the tank set up and have it cycled properly. It will take many weeks to cycle the tank, and you will have plenty of time to think about what fish to put in there. In case you change your mind on getting a betta fish, please let us know.
