Rocky
Yes you can mix the mat and gloss acrylics together, but it will be glossy depending in the ratio of mat to gloss, eg: 50:50 will give a semi-gloss.
Caveats:
Always mix both colours well seperately.
Use different pippettes/coffee stirrers to transfer to a clean mixing jar.
Mix MORE than you think you need, as if you don't you will always have to mix more... 
Use of X-21 Flat Base as directed, (a gloopy paste used to matt out Tamiya gloss), will even out the tone, if flat cote is needed. This isn't always needed, & I rarely use now.
If decalling, glossier is better though.
Anything you add to the paint, thinners, flow enhancers, etc. will alter the paint tone btw...
Use X-20A Acrylic Thinner, to thin a little of your mix, spray, & allow to dry, as the tone/shade will change a little as it dries & it will depend on the underlying colour of plastic/primer...
The concensus opinion is that few paint shades are accurate anyway, & your base coat should be just that, your starting point...
Have fun.
What colour are you aiming for? could be other paint ranges have the colour or shade you are looking for already?