Kev's given some great advice, but I think you'll find even within our household we'll disagree on a few fry raising techniques!

(I'm his fiance)
The babies at this age need to be fed more like 4 times a day. Over on the org they recommend siphoning out whatever is not eaten, but I have had better luck with leaving the bbs in for them to graze on all day and then doing a one time a day large water change.
If your filter is cycled and you're not getting any ammonia or nitrite readings then no need for the amquel+. amquel+ can really decrease the oxygen in the tank anyway, so it's best to not use it if you don't have to.
Beth's seahorses are southern so Kev is right that you don't need a kreisel. However some people like it and prefer this method of raising them. We have tried both and the regular old 10 gallon method has seemed to work best for us. It's all in whatever works best for you.
I would recommend not doing live rock, chaeto, etc. Fry need sterile tanks. Even the tiniest of hydroids can take these guys out very quickly. Treat the tank like it's for dwarfs. If you do decide to go the live rock route, probably best to start with dead rock and make it live yourself. If you do add chaeto, panacur it first.
Like Kev said the sponge filters are really nice with fry. We have done the power filter thing with sponge material over the end, but seahorses still find a way in. However, with them in a kreisel they're not going to find a way to the power filter. If you decide to ditch the kreisel that's something to think about. You could float a sponge in the tank now to seed it.
As for changing the filters, change em as often as you think. If they're getting really dirty then change them. If they're not then leave them. It's all trial and error and there is no cookie cutter fry raising setup for seahorses.
I assume you're hatching BBS daily. They're pretty much nutritionally worthless after 24 hours, so best to hatch new ones daily. You can even stick them in the fridge after draining the hatchery to slow down their metabolism and keep their yolks bigger, longer.
I usually start adding frozen cyclopeeze as soon as one week because I have actually had them start eating as young as one week. But that is only with a cycled filter, because the frozen will crap up the water fast!
Good luck and keep us updated!