Sun sign is not an indicator of compatibility. We'd get more information from aspects involving your Moons, Mercurys, and Venuses. Mars, too, especially if one of you has it in aspect to the other's any of the above.
If your Moons are trine, sextile, or in the same sign, you're both on approximately the same page regarding deep emotional needs, security, and what you need/want in the home. You don't necessarily want the exact same thing, but you have similar ways of approaching those things, and each other's ways will make sense to you. If they're in square or opposition, you would have very different approaches to those things, and to have a successful relationship, you would have to both recognize that and work on it.
Same thing with Venuses, only there, we're talking about how you do love and relationships. Mercury would indicate communication: do you communicate easily, in similar styles (harmonious Mercury aspect)? Or in very different ways, which require a lot of work to understand each other's points of view (hard Mercury aspect)?
Ideally, there should be both easy and hard aspects involved in your synastry. Too many easy ones, and it becomes a boring, placid relationship with little impetus for growth. Too many hard ones, and having the relationship is extremely challenging. Hard aspects show where you have to work at understanding each other.
It's not even that simple, though. To do full on synastry reading, we'd also have to look at other ways your charts aspect each other's. For example, if one of you has Moon conjunct the other's Pluto, that speaks to a certain deeply unconscious power dynamic that could arise, for better or worse (and if you're the same age or nearly, one of you having Moon conjunct the other's Pluto would mean Moon conjunct his own Pluto, so would trip whatever Moon/Pluto issues he's already working with).
So, short answer to your question: a Pisces could be compatible with your placements. It depends on the rest of the chart.