It is best to remove the old tiles if possible. Adhesion is not brilliant onto old tiles, even when the glaze is failing. Bear in mind that these new tiles will be on for another 25 years and any imperfections are a real pain. That said, if the existing tiles are perfectly well attached and level then you might want to risk it. Use a powered wire brush to remove all the crap and leave the best surface you can.
The glass mosaic tiles are great. We got the last lot from Bauhaus. Don't try to fix and grout in one hit. You need a good layer of adhesive and you need to concentrate on getting the new tiles level using a longish straight edge. If you try and worry about grouting at the same time then you'll end up with wavy tiles. Grouting is much easier onto tiles that are well fixed onto fully dried adhesive.
One other point. The glass tiles are very bright, perhaps garish. Check that the colour you choose won't stick out like a sore thumb.