There are limited studies in the literature on the effects of radiofrequency and fillers. The one animal study that exists suggested no reduction in the filler volume, however an inflammatory response around the fillers (which would lead to increased collagen formation). Venus freeze should not "melt" the filler but may have some effect on it. There are anecdotal reports of reduction in filler volume with other treatments such as deeper laser treatments. The massaging action of the Venus freeze handpiece could, however, help to smooth any palpable lumps or irregularities in the very early stages following filler injections.
With no clear evidence available, the best way to stage your treatment would be to have the skin tightening procedure first, followed by filler (you may even need less filler). The other option if it is early after your filler, to avoid the areas which have been filled eg, if you had your cheeks filled skin tightening could still be achieved below and around this area.