No, you can't use stearic acid or cetyl alcohol. They will eventually separate out and leave a gooey white mess at the bottom as they are not water soluble. So you'll want to use something water soluble like glycol distearate or PEG-150 distearate. Crothix was developed specifically to thicken surfactant blends, and it works really well (sometimes too well, creating Jell-O like substances! I suggest using the liquid because it's easier to control than the pastilles!)
You can use hydroxypropyl methylcellulose. As I've used this and wasn't really happy with it - it precipitated to the bottom, so I know I've done something wrong - I'd suggest consulting your supplier to find out how to use it. You can get pre-blended mixtures of this stuff (click here for more information from the Personal Formulator), which you would add at 5% in your surfactant mixture.
You could use something like carbomer for making gels. Here's a post I wrote on the topic a a while ago. Or you could thicken it up with some water soluble oils - they won't thicken as well as the other things I've mentioned, but they will increase the viscosity. Or you could use oils with polysorbate 80 in a surfactant mixture (like this body wash recipe here). Again, this won't thicken as much as the other suggestions, but it will thicken it up slightly.
You can also use salt to increase the viscosity of any anionic surfactant blend - this is called the salt curve. This is a tricky business and easy to mess up. Just add salt a bit at a time until it reaches the thickness you like. (Look for a post on this topic tomorrow morning...)
You can increase the viscosity of your surfactant mixtures by adding an amphoteric surfactant like cocamidopropyl betaine (I include this in all my recipes to increase mildness and add thickening). You can also make a thicker surfactant blend by reducing the water and increasing the surfactants. This will be more concentrated, and you'll need less. And you can use thicker surfactants. A mixture of something like SLeS, Plantapon, and cocamidopropyl betaine will be quite liquidy, while a mixture of something like BSB, LSB, and cocamidopropyl betaine will be quite thick.
You can also thicken your mixtures by including something like cocoamide DEA (I've been playing with this, and a post on this topic is coming shortly. See the picture to right - yeah, it's not pretty, but I stink at taking pictures!)
And finally, you can thicken your surfactant mixtures by choosing your essential or fragrance oils wisely. (Click here for a post on this topic). I have found that using a 2% mixture of half lavender, half rosemary thickens my bubble bath enough that I don't need to include any thickeners. Citrus based essential or fragrance oils work really well for thickening.
As a note, I'll be taking a closer look at surfactants in March, so keep a look out for that!