Yes, you can use Klear (Future, in the USA) as a gloss coat before decals. It's often better to brush this on, but look at the various discussions on this, elsewhere on the forum, to see what's needed and what should be avoided. It works well, and is simple to use.
However, the final clear coat, in this case a matt coat, would need to be acrylic also, as solvent-based varnishes would react with the Klear, and Tamiya Flat base can be added to Klear to make this, although testing for the ratio of mix must be done first. Again, this is discussed in a number of threads here on the forum, as are many subjects which pop up repeatedly.
I've used Humbrol enamels for over 50 years, which used to be superior to Revell by a long way, and superior to most other paints by some measure. But, until very recently, they have been made elsewhere in the World, since the demise of Humbrol as a company some years ago and were, quite frankly, cr@p !
I had to persevere though, as they are the only paints I can obtain locally, although mail order for Xtracolour has saved the day some times. But I stopped using Humbrol Matt Coat many years ago, as I found it inconsistent, being either too matt, or edging towards semi gloss, plus, of course, the problem of yellowing with age.
I first changed to an acrylic varnish, and then discovered Klear, and the ability to mix with the FB, to give both gloss, matt and everything in between.