You can certainly build decent fillets with colloidal silica. You may wish to consider collodial cellulose as an alternative, though. Colloidal silica is a respiratory hazard (silicosis), so you need to wear a respirator while mixing and grinding it. Also, once the epoxy hardens you can't sand the colloidal silica based fillet - it's too hard. You have to use a grinder on it. Colloidal cellulose is the same color and doesn't have these problems.

I'd get about the same amount of colloidal whatever, by volume, as the wood flour.  

   I frequently mix cellofill and wood flour to match whatever wood color I'm fileting.  Usually about 2 or 3 parts cellofill and 1 part wood flour for okume, maybe 1 part / 1 part for sapelle.  Remember that the wood will darken when the first coat of epoxy goes on, while the filet won't get any darker.

And having some cellofill in the mix seems to allow you to get a smoother mixture even up to the point where the filet constistency is at least as thick as peanut butter.  Wood flour only mixtures start to develop "surface texture" when you are tooling a really thick mix.  Nothing that an alcohol-finger rub after partial set-up can't fix, but there is less need for smoothing when there is some cellofil present.  And because wood flour alone is almost always darker than any of the wood we're working on with CLC kits, I'm always adding at least some cellofill.

I believe it is slightly harder to sand mixtures that include cellofill than the pure wood flour filets, but I've learned to do careful filets, and post-filet clean up and smoothing, such that I almost never need to do any significant sanding on or around them after the epoxy has set up.

I've even mixed alcohol-based penetrating stain into the mix to make filets match the stained wood. It doesn't take much stain.

And like Chenier said, I'd stay away from silica.

Colloidal silica's a fine epoxy thickener for structural applications like laminating wood to wood or plywood.

Take it to heart it's a health hazard when inhaled, whether as the raw product during mixing or as dust produced from sanding or other post-cure work. With epoxy as well as the various fillers employed, proper PPE is a Best Practice!!

Compared to the very viable alternate of Cellofil - finely milled cellulose fibers - silica's also quite a bit heavier so there's that if you're concerned at all about weight of your project. And it resists most any effort to sand it. Cellofil's more forgiving in that aspect.

As other suggest I've used Cellofil in combination with wood flour for filleting, to gain a smoother surface. As a fine filler when added to wood flour (not unlike the small sand particles added to larger aggregates when mixing up concrete) the effect is to fill in between the wood dust bits. And it's a fine filler for mixing epoxy for laminating too, keeps the stuff from being squeezed out under the pressures our clamps bring to the work.