The trademark aggression of bettas is generally between males. Males and females can be paired but you will never know if the male will accept the female or reject her. However this is no issue here, as the pair was for the kribs I believe. Bettas, like gouramis, breed in bubble nests constructed by the male from bits of plants and his own bubbles.
I'll try and work out if this would cause over stocking using "one inch of fully grown fish per gallon" which, though contraversial, does help to stock a safe amount of fish:
7 zebra danios, 7inches, 7 gallons.
1 pearl, 2 inches, now 9 gallons.
1 guppy, one inch, now 10 gallons.
2 platies, 4 inch, now 14 gallons.
1 betta, 2 inches, now 16 gallons.
2 kribs, 8 inches, now 24 gallons.
6 (nice shoal number) tiger barbs, 12 inches, now 36 gallons.
1 BN pleco, 6 inches, now 42 gallons.
6 dwarf rainbows, 2 inches, now 54 gallons.
From that (the ones you didn't specify numbers for I put the ideal or minimal situation), you're just in in terms of stocking. HOWEVER;
Bettas + tiger barbs DO NOT MIX. The better's fins would become breakfast for the barbs. I'd suggest either replacing the barbs or the betta, personally I'd go with the barbs and buy some of the less nippy types, there are quite a few, or maybe have a larger shoal of rainbows, if you got 12 overall that'd be a lovely shoal.
