The only soil I use is just the plain miragle grow. But I don't have the problems you do with a drier climate. I think if you vented covers on all of your containers, it might help with that. Since I've never used the soil with crystals, I'll let someone else with more experience chime in.
Nice coleus by the way. I've heard that's temperamental to winter sow. Nice going! :)
There shouldn't be any problem using hydro crystals when you ws, if you don't get very much precipitation in your area. They will definitely help to keep your soil moist. Do remember to put at least 4 inches of good potting soil in your container. Using less soil will make your containers dry out to quickly.
Lucky you that Coleus can be ws in your growing zone!!! Unfortunately, I don't have that ability in my area and have to start them under lights each year. I'm also trying to over winter several I grew last season. However, they do tend to get a woody stem, but that's okay. They still have their beautiful & colorful foliage.
I've been using the polymer moisture crystals in all my containers for a few years now... it really helps keep things from drying out, and I think it also helps to keep soil mx from being too soggy also.
Last year, I used the small size crystals from the Watersorb folks, http://www.watersorb.com/application.htm in my seed starting flats (both wintersowing and indoors) and in my seedling cell packs. I think it really helped, and I liked the smaller size because the little crystals are less likely to toss things around in small containers when they expand.
bad news... watersorb quit carrying the small size crystals! :-(
It seems people kept ordering small crystals instead of medium when making "cool ties" to send to our troops, and after lots of frustration (how many times and ways can you tell people to order the medium crystals for cool ties??!?), they decided the best thing was to stop offering the small crystals. Waaah!
I'm going to see if there is some way to pulverize the larger crystals to make smaller ones... I may also experiement with adding some of the powdered polymer (which I have to use on bare root plants and for shipping plants) to my seed starting mix. There's gotta be a way!