A small number of units is not always a bad thing, especialy when you are learning the game. First of all the black sun is the most versatile force in anima tactics ( took it from wissenshaft & samael). You don't have much troops now, but they can be customized in a variety of different ways. Its not just about learning how to use the unit in a variety of different ways. You use different buids of your unded units and learn how to use it. Its big advantage in the long run beacause most players stick to few learnd tricks with some characters and with each new buy they lose some of the learning curve.
Mainly you don't see many Church players using Grey or Luzbell, or wissenshaft fielding Veronica or Feng yi. Even Cellia gets left out most of the time in favour of some power builds. Its not that this units re bad, but they are difficut to use.
So stick with the releases you got and experiment as much as you can. A really crapy model can become the center of your strategy if you know what to do with him
When you get to know. What your black sun lacks, look at the wanderers. They are mostly used to fill the gap. If you lack offensive, defensive support , pick Sophia, if you want one powerhouse model, go for Dereck or Faust. If you need speed , take a look a the three drgons, For Tanking lady Khaine is your nwser. Stealth and anti armor is governd by takanosuke and Kujaku Hime. Finally if you need some versality and fighting prowess choose Bella or Drake & Cennet. They often included for their great abilties and will never let you down..
I wouldn't consider summoners for the beginers. Its mostly Samaels and Chrurchs field of expertise, and the only valiable wanderer summon combo right now is belith & ahriman/al dijin. The first one being a resonable option and the second one used mostly for crazy ash slave combos that can be very complicated at this point.
The main point of this advice is to stick to the basics and experiment. Don't use the obvious combos. Victory will teach you nothing but failure a thousand things about your playstyle and oponent.