What's legal use depends entirely on the specific license. So there's no way this question can be answered without looking at that.
If the license isn't something you've encountered before or understand the terms of well, really this isn't a call you should be making. Your employer should have a licensing specialist and/or a lawyer for this exact purpose. You should take the license to them, explain exactly how you want to use the software, and get a decision from them.
This is why its really helpful when software comes with standard well-known licenses. Any time I see something that isn't something common that I know well (CC0, GPL, BSD, Apache, etc.), honestly its easier to just find an alternative that is.
One resource that can help newbies is the GPL License Compatibility list. In general, you are always free to use GPL software for anything you won't distribute outside your company*. If that's the use-case, then this list is really good for showing you what other licenses are compatible with that kind of use, and where they may or may not be problematic. If the license isn't on this list, I'm not sure I'd trust it. Certainly wouldn't without reading and fully understanding it first (and really, that time's usually better spent finding an alternative online).
If the software isn't for internal use, but instead you intend to distribute it outside your company, then pros really need to get involved.
* - Not to imply its impossible to distribute software that uses the GPL. Lots of companies do that. However, your license must also be GPL compatible, which your entire sales strategy really needs to be built around. That's not something most companies are going to be amenable to doing just because you found a nice free tool online.