Big to small

As someone that did what your suggesting. I would not recommend it. Any of the Aneros line would fine. If your on the fence about putting something up there go with the Eupho. If ready to commit go with the Helix or MGX. The Eupho is too small that unless you know what feeling will cause to O you won't know if it is stimulating you correctly.

Once you get comfortable with the feeling of one of the Aneros products, I do feel something larger up there may help if your struggling. Make sure it is specifically a prostate massager or curved as a regular butt plug won't put enough pressure on your prostate. 

The upside to an Aneros product is they are small enough to not give you the feeling like you got to poop but the downside is that unless you know exactly how the Aneros should be moving it to focused to give you a prostate O. The bigger products are less focused which gives your short and weaker Os but at least helps train your mind to what the feeling is. The smaller ones focus more and gives your more stimulation with a bigger/longer O but are harder to get there as if your miss or squeeze incorrectly it wont stimulate at all.

For me I alternated between a larger vibrating prostate massager and a Helix(Non Vibe) as the prostate massager would give me bigger/quicker traditional Os. I would go for 10-20 mins before giving up but finally I got a super weak prostate O with the massager and I switched back to the helix and I was unlocked and knew exactly how the Helix should be moving.

At the end, I do believe that switching up the sensations down there do help but with the larger prostate massager that are sold be every one do give you the feeling like your going to crap and can cause it to be hard to keep in there until your body gets used to that sensation.