Whether a toy will get sucked in or not should be a simple thing but there are actually quite a few different factors at play, which all interlink. It is possible for a toy to be bad in one area but make up for it elsewhere.
(Some of these aren’t directly relevant to the plugs you’ve bought but I’m including for others reading).
Ultimately the most important thing is that the base needs to be large enough for the toy and for you. For the toy, this means that it should be bigger than the widest point of the toy. The safest options are 1.5/2 times the size of the toy but this isn’t always possible with round based plugs. However, the bigger you can get the better and don’t go for anything with a base smaller than the rest of the toy. For you, this means comparing the size of the base to the size of toys you can take easily. If you can insert toys that are bigger than the base of the plug without thinking about it (ie no warm up, minimal lube) then you are more likely to have issues with the plug. If the plug in question took a little warm up to go in (and the base is the widest part), you are less likely to have issues.
The base needs to be firm, anything too flexible will just fold in. (Not an issue for metal and glass obviously)
The base shouldn’t taper into the neck ideally. Tapering helps the plug go in so can increase the chance of issues. (Most jewel plugs have fairly defined neck and base though). Having a distinction between the bulb of the toy and the neck will also help.
Different materials will change things. Silicone will have a little more grip to it, especially if you go for one that isn’t the soft touch type silicone finish (although all silicone will grip). Glass and metal have more glide and therefore move a little easier.
What you are doing. Using during penetrative sex will be different from using while masturbating. Wearing in the gym will be different from wearing around the house. Obviously during orgasm, muscles are being contracted so that will make sex toys shift.
Individual anatomy. Everyone’s rectum has the power to suck small toys in but some are more powerful than others. Everyone also has different tension in their sphincter so some people will clench toys easier/tighter/with less thought.
Be a bit careful what you are doing. Don’t push on the plug.
In short
A big enough base is the most important bit. If you are doing something more active/involving having orgasms and you want a round based plug, go for one where the base is bigger than the widest part of the toy and that the plug is a comfortable size but still takes a little effort to put in.