I wouldn't pay $15 for te whammin watermelons. That's the most common morph and $15 seems a little steep for 10 polyps. The trumpets are a great deal and the bam bams are a little pricey too but that's not unheard of for them
I would wait on the slimer. Acros, even the easy ones like green slimer, shouldn't go in for a couple of months. That gives things time to stabilize and let you play with keeping calc/alk/ and mag in check. It's a beautiful coral and grows like a weed in my system. I'd be scared of having it in a tiny 5. Guess you can frag it easily enough though. The other corals sound fine. What's your lighting again? Here's my slimer to give you an idea of it
I, and many other members, have the last fixture. it's capable of growing any corals, but it could be an overkill for the Chi
Wavepoint clip?
You will need good lights for the mystic sunset. It is an encrusting montipora, an awesome one at that, but an sps coral too. The par bulb or the 120 dimmable fixture will work but not the other. On the caluerpa don't put it in your tank. It will take over a 5 gallon in several weeks. And if it goes sexual will nuke your tank.
Haha I know how you feel. Everyone has differing opinions in the hobby. I agree the mystic would do fine at the top in moderate lighting and good flow. The half blue white polyps are awesome. Look up modified aqua clear hob fuges. That will let you grow the chaeto in it as a semi refugium. On the caluerpa you could try it in a jar or on the sand I just wouldn't let it get on the rocks. With your coral list growing your gonna need that 35 gallon sooner than later . Duncan's grow incredibly fast but don't take up much room. Mine went from 2 heads to around 40 in a year and a half lol.