Pjur Eros is safe with both the Decadent Indulgence and the Xtreme Pack, as neither of these is made of a silicone-incompatible material (TPR and Plastic are both listed as being safe with silicone lube).
Depending on which variety of Pjur you have, it actually might be safe to use Pjur Eros with your Gigi, given that they're both very high quality silicone (original is safe with SOME silicone toys, but I don't know about the rest). Still, I'd be careful to do a small patch-test first. You're very right that most of the time, it's not safe to use silicone lubes with silicone toys, but when both silicones are very high quality, as with Tantus and Pjur (they are advertised as being safe to use together, which is why I mentioned it), they play nicely.
How much you need for anal use depends on what you're putting back there, and how experienced you are. When I use Pjur Eros vaginally, I use a nickel-sized dollop. For my smooth butt plug in my rather inexperienced ass, I use half again to twice this amount, and I keep the bottle on hand to re-apply if necessary. Anything with more texture and/or more thrusting will probably require more. When in doubt, put down a towel before you get going, and over-apply. Yes, it's an expensive lube, but it won't take long to figure out how much is right for you.