As I mentioned in my Select Seeds post, I did not buy plants from them (yet), but my guess would be that you will receive small plants for this price. According to Garden Watchdog comments, the plants they send are healthy.
There is nothing wrong with small Clematis, they will catch up in no time.
Pot them up into 1 gallon or larger pots and keep them potted till fall. Meanwhile, prepare your beds. Make sure that the crown of your Clematis is below the surface of the soil (ideally, 3-4", but for temporary planting it can be less than that). Keep pinching new growth to get a bushier multi-vine plants.
I do not think it is a good idea to pot two Clematis in one pot. Your goal this year is stimulating root growth. Deep planting and pinching the shoots will do this.
If you will plant two or more Clematis on the same bed, you should opt for Clematis in the same pruning group. Planting group II and group III together can make future pruning problematic.

I actually wanted to mix them on the same trellis. If I plant them on opposite sides will they have enough room? 18 inchea apart?

Yes, that is fine. Again, make sure you do not mix two pruning groups.

I checked, both group 3. Chop away. :)

Group 3 is generally better than group 2 in terms of performance, so you made a good choice for your first Clematis.
Good luck!

Make sure your trellis is big enough for them. :)

I use alfalfa pellets from the feed store and rose tone to feed mine plus compost mulching.

Yay! I hope the one I planted last yr blooms soon. It's taking off like a rocket. Say, does anyone know where to get a reallytall trellis? Most of what I find is only 4-6ft. May have to improvise.

What you feed is dependent on your soil type, i.e. if you have clay, rocky, rich, etc... how fast your soil drains and what minerals make up your soil. i.e. is it high or low pH, calcium/magnesium based... generally you can tell your pH level by your water hardness, soft water means low pH, hard water means high pH. You can always add organic manure, mulch, peat moss etc... you're only going to have 1 shot at preparing that bed. I always toss in lime (calcium/magnesium grit) in mine because I'm in the North East, and a healthy dose of the cheapest 12-12-12 fertilizer when making a bed. Then each season I mulch and a couple times a season I add the slow release fertilizer in pelletized form because it's hard to burn with those granules.

I'm pretty new to clems, but from readings, where they do best (i.e. england, northwest US etc...) they like a rich soil (high organic and nutrient) and plenty of water, as long as it drains. So fertilize away as long as you have good drainage, if you have clay, dig down with organics as far as you can, even just a bag of mulch mixed in to increase the organic matter can help (although not too much non-decomposed matter as it can rob nitrogen initially).