Can I use guar and gelatine together?

You'd have to use a whole lot of gelatin to ruin the taste. My guess is that when you experienced that in the past, you were using (perhaps unknowingly) flavoured gelatin or "dessert gelatin" instead of ordinary, pure, unflavoured gelatin crystals or sheets.

Erik is correct in that gelatin does not do well with tropical fruits (including mangoes), nor acidic environments, nor alcohol, and you have all of those things. Stay away from gelatin. A much better idea here would be Agar (also "agar agar") - again, make sure it's pure agar, not the flavoured dessert stuff - and use about 0.2% agar by mass. You need very little of it compared to gelatin, so it most definitely won't affect the taste. In fact, I might go as low as 0.1% because you don't really want setting action, just thickening. By the way, make sure you hydrate it properly; agar needs to be boiled before cooling in order to set.

Of course one of the nicest things about gelatin is the fact that its gelling action is reversible and it has a melting point of around mouth temperature, so gelatin desserts or fillings will literally melt in your mouth. But if you combine it with guar gum then you'll get a lot less of that anyway since guar is not thermoreversible like gelatin is.

Xanthan gum is another interesting choice due its shear-thinning property. What that means is that it won't literally "set" like agar or gelatin, but will hold firm as long as it's not moving. That can get you a little bit closer to the melt-in-your-mouth feel of gelatin, but you also risk having it all ooze out while you're slicing the cake, or fall apart on your fork. It's really more useful as a stabilizer than as a gelling agent, but I thought it worth explaining in more detail since Erik mentioned it in his answer.

Any of these things can be used with guar gum, corn starch, or any other traditional thickener; just don't use too much or else you'll end up with Jell-O (or worse) as your filling.

By the way, another thing you can use (which is quite common in fruit fillings) is pectin crystals. The most common kind of pectin does well with high acidity and sugar, which I believe is what you want, although I'm not sure about the acidity of your filling - you'll need quite a bit of acidity (around that of orange juice) and quite a lot of sugar (60-80% according to Khymos). It's a little trickier to get this right, and I'm not 100% sure about any possible negative interactions with guar gum, so I'm leaving this as more of a footnote than a recommendation.

To me, the definitive guide to all these gelling agents is "Texture", the free e-book at khymos.org (which I know about because of this site, by the way). It says that mango is an inhibitor to the working of gelatin, so gelatin won't help as much as you might hope. Having said that, some of the example recipes do use gelatin, so it might still help enough.

About guar gum, it says that acidity (low pH) is an inhibitor; are there any acids in your mango puree? It also says that guar gum leads to high viscosity when warm and low when colder, so cooling it down should definitely make things more solid. You could also try some of the other agents; maybe xanthan gum will help.