If a utensil is safe for a 'normal' microwave, is it also safe for a flatbed microwave? I have some silicon based utensils (a steam case, a rice cooker and one for cooking pasta). These are developed for use in microwaves and I do not want to get rid of them. However, I'm planning in substituting my ancient turntable microwave with a new modern flatbed microwave.

Is it safe to assume that utensils apt for the turntable microwave are also safe in the flatbed one? That is, provided that you are using the same power setting.
Yes it is safe, all microwaves work on the same principle, and use the same range of radio frequencies to heat food. All microwaves used to be flatbeds, most commercial microwaves are still flatbeds, so you have nothing to worry about.