Only just found your threads, sorry to see no one has replied to you
I wanted to get into sisters but price limitations got me down (what's a teenager to do with no job?) but before I decided it wasn't going to happen I did a fair bit of homework, so I should be able to give advise as good as any on from this site.
IF you are up for using proxies, then maybe go the whole shebang! If you are allowed forgeworld (which by all rights, you should be) then use rhinos as Repressors which are basically rhinos with AV 13 on the front, which is AWESOME! I see your point on storm bolters over metlas because its obvious they are your objective scoring troops and wont be in melta range, but even so a stormbolter is never worth it. Take a flamer to give some nice overwatch and some viability to push other units off objectives. Maybe swap a sister squad for a dominion squad with double melta in an immolator with multi melta to scout forward to handle AV 14, which you have no answer to in your army save to glance to death, but you may find yourself low on troops then.
MSU's of sisters doesn't work too well I'm sorry to say and I'd be hard pressed to advise those Seraphims. A big unit of 9 would be more then enough. The reason being that my guess is that you plan to deep strike them as suicide units, because running them up the field would be suicide anyway. I find deep striking with template weapons as a bad idea in general without a way to control scatter. But a unit of 9 with Celestine running at the front to tank can take a bit of fire on Celestine, who then gets back up for more! Celestine has No unit to be running with at the moment, not that see especially needs one, but the Seraphims Act of faith helps her flamer and she helps them pass it and tanks fire.
triple Exorcist is ok I guess, but I'm seeing a lot of people running Retributors with heavy bolters because of how good they are with rending. I would suggest dropping an exorcist for such a squad with 4 heavy bolters and REALLY trying to make points for an ADL with quadgun. The superior would man it and it would benefit from rending making it a good anti flyer option, of which you have non.
hope I could help.