So the first piece of advice I can give you is to cork your liqueurs tightly. Get a corking device, get good quality corks from your local wine-making store, and either get boring, standard wine bottles or be careful about the necks of the fancy bottles you do buy. The only way oxygen is getting into your liqueurs once you've bottled them is through the neck, so be careful how you treat it. Of course, if you're only going to be storing your liqueurs for as long as it takes to age and drink them, you can certainly afford to be more lax in your efforts. They are well worth the investment.
Most unusual bottles come with brittle, porous, tapered corks.
They are pretty much useless for actually sealing bottles. Beyond the obvious hazards of stiff, brittle and porous material, the tapering angle is probably the most serious concern. What makes wine corks good is that they're cylindrical - straight from top to bottom. Compress a cork into the neck of a bottle, and it's an inch and a half of solid seal, in complete contact with the glass. Even if there are many slight imperfections, as long as they don't form a continuous chain, you're still sealed.