3 Congo Tetra = 9 inches
3 Serpae Tetra = 6 inches
6 Neon/Glowlight/Black Neon = 12 inches
1 plec = 5 inches
That gives you 32 inches of fish, which is a little overstocked technically speaking, but nothing that can't be managed with an extra
water change or two. Since you're going to be on the heavily stocked end of the spectrum, planting is a great idea, as you mentioned, to keep the nitrates in check. I hear crypts are really easy, but I've had mine less than 24 hrs so I can't give a personal opinion lol. Best of luck, and be sure to post pics!