Rick was finally ready to fight back against Negan and the Saviors once The Walking Dead returned following its Season 7 hiatus, but he was at a serious disadvantage after losing so many supplies and so many people. Rick needed weapons and, more importantly, he needed an army. But when he appealed to both the Hilltop and the Kingdom for help, he was turned down; for those communities, it wasn't worth the risk to fight back. They needed to protect themselves instead of going on the offensive. However, Rick's luck seemed to change when he was able to strike a deal with the Scavengers, the new community on the scene, but can Rick trust the Scavengers on The Walking Dead?
Having only met the Scavengers and their leader Jadis once, there is no way to know just how trustworthy they might be. There is still so much that the audience — and Rick himself — doesn't know about them. They may have given their word that they would help Rick in exchange for repayment in the form of supplies, but there is no telling whether they're the kind of people who keep their word or not. However, there might be some context clues that give insight into how trustworthy they may or may not be.
The fact that they mostly keep to themselves indicates that they put the interest of the group ahead of anything else, and Jadis insisted that while they may steal things, they "don't bother" other communities. How kidnapping Father Gabriel fits into that is unclear. Taking people seems kind of like crossing a line into bothering them, but I imagine the honor system of a junkyard scavenger gang is tenuous at best.
Rick may be able to trust the Scavengers as long as they continue to benefit from their deal, but that support is not necessarily unconditional. They're essentially hired soldiers. They're helping Rick for the eventual payoff, not because they're loyal to him like his own people are. So while they may be helpful going forward, Rick will have to tread carefully.
Their alliance is still in its earliest days, however, which means there is time for the relationship between the Alexandrians and the Scavengers to grow stronger. Rick seems to win over almost anyone he comes across (those who don't end up Team Rick don't usually last for very long), so the Scavengers could end up being genuinely loyal to him. Either that, or they could end up being just another problem to deal with.