Tank is great for adding more fish and it sounds like a plan as far as the plants go. Should be gorgeous. However, I would add no more fish until it is better planted. Keep in mind what I said: They still see your boy as a predator and need the security not only of a larger shoal but of more plants which breaks up the lines of sight and chase.
Do not expect that because your Betta is fine now he will remain that way. Yes, normally they do; but they are unpredictable fish and all of that can change overnight.
Use SeaChem Stability when adding new fish to even a cycled tank. It helps compensate for the added bio-load;
Use two filters instead of one. I have found people often forget that tank mates do not have labyrinth organs so they need well-oxygenated water. As you have a Plakat, you don't need to worry about too much current...which is good for tank overall health and great exercise for your Betta. Surface agitation prevents slick biofilm from forming. Oh, the other thing about two filters is if you are *ever* tempted to get another tank you have a cycled filter ready to use....This, says my husband, is the real reason I run two filters in my larger tanks.
Feed everybody the same thing. Too many, IMO, obsess over feeding the Betta this and one tank mate that and another that. I feed frozen, Fluval Bug Bites for small fish, same for medium-large fish and NorthFin Betta Bits.
As an aside, I am always saddened when I hear of forums and FB groups that are so judgmental....especially when so many "experts" are just parroting what they've heard and not basing their statements/advice on real-life experience.
This forum is, IMHO, special in that we help people with what they have and while we may suggest changes we do so kindly.
[B]Clarification:[/B] After thinking about it, I feel I should add that there are a lot of very knowledgeable people who have not had a lot of experience. Their advice is sound and spot-on. I was only referring to those who are dogmatic, short-tempered and unforgiving if you don't do things their way. So, I apologize to those members who fall into the first category. Your contributions to this or any other forum are immeasurable and much appreciated.