Sure, Dullcote is fine if you're talking about the level of sheen.
If you're using Polly Scale acrylics, then putting Dullcote over the top, make sure the acrylics have had at least 48 hours to dry first.
Although I'm not a paint expert, here's what basically happens if you don't...
If the acrylics aren't fully cured and hardened, they're still soft underneath and adhesion to the surface isn't that great. Applying an enamel over the top, like Dullcote, prevents the acrylic from drying completely. As the enamel dries, it shrinks, and the adhesion of the enamel to the acrylic is stronger than the acrylic to the model's surface...the result is curdled, cracked paint.
I just answered a similar question in another thread about in expensive way to flat-coat, provided you have an airbrush... so I'll just re-post it here for you.
Trod on down to the local supermarket and buy a bottle of Future, then
hop on over to your local hobby shop and buy a couple of bottles of
Tamiya X-21 flat base.
Mix the two together, I generally start at a 4:1 ratio of Future to X-21.
Once you find that ratio that you're looking for, mix up a big batch (for consistency's sake), and spray away.
Gloss coat the model before decaling with regular Future.
This way, you're not applying dis-similar flat coats over each other, and you're less likely to get reactive results.
If you must use a rattle-can, look for acrylic clear flat sold in craft stores. Artist's use it on charcoal and pastel drawings to prevent smudges. Experiment first before putting it on your model, just to make sure it's compatible (that's the official disclaimer).
Jeff