I'm glad other said it too and that you've discovered you can grow more than just Java moss and fern in 1wpg! I've got PLENTY of good growth going on in my tanks and 2 of them are LESS than 1wpg!
Also glad you went along with your Betta sorority idea, male bettas are obviously a no no together, but females just need to establish that pecking order and they're fine. If you still wanted to add more fish though I'd advise against guppies, at least male fancy guppies, because Bettas tend to see the fancy fins as a a sign that the fish are just weird looking Bettas. I've never had much success keeping male guppies with male or female Bettas.
As for the algae, dosing with excel and some trace fertilizer (like Flourish comprehensive) should help the plants out-compete the algae. Also, you might think about adding some Hygrophila difformis (water wisteria) to your tank as it's always grown pretty well and FAST in my <1wpg tanks, that might be a good nutrient sponge to help combat the algae. Spot-treating with excel can work too, but might get costly / lead to an overdose if you have algae everywhere. Or some shrimp, amano or adult RCS should be ok with the bettas. I put 5 juvie (1/4inch/5mm) RCS in my community tank with guppies and zebra danios and they've all grown to adulthood just fine, and it looks like you've got some nice foresty areas that they could hide in which helps.
Ok I think that's all I've got for now
Did I mention the tank looks great though?
I'm impressed with the growth in 2 months!