Im riding 18in rims right now they dont rub they fit well.but now i want to go 16in rims.they are 225/50/16 all round.they dont rub at all.so i like 225/50/16 on the back but front not sure if i should change tyres to 215/50/16 or 215/45/16 or 205/45/16 ???

For an MR2, a 205 front with the 225 rear would give the right stagger, assuming you fit the same brand/type tyre F/R. 
What are your rim widths?
If your rim width is over 7.5" you won't be able to fit a 205. (or a 215 for that matter either)   
 

Tyre place told me if they not stragger rims dont change the size of tyres not good.stick to one size 225/50/16 all rounded.

Unfortunately they dont seem to have much of a clue, you'll have to get used to this on all ends of the car related spectrum when it comes to MR2's, this is why so many of us learn on the internet and do our own work. MR2's love stagger.
 
I run 16x7+38 195/40/16 and 205/40/16.  In theory I should have run 215's on the back but I've been real happy with that combo. In hindsight 50's would have given a better ride but oh well.

Richieyellowmr2
Tyre place told me if they not stragger rims dont change the size of tyres not good.stick to one size 225/50/16 all rounded.

 
What place said that? Totally carp advice.
 

Jax tyres

I have a set of 16" 7 and 8inch stagger Linea Sports available

Richieyellowmr2
Jax tyres

Jax off?
Which franchise? not fair to tar the whole lot with one brush - they'd be independently owned I bet.

Thanks heaps for advice i go ahead keep my rear at 225/50 and get 215/50 at front.and get all the same brand if possible 😊

When selecting aftermarket tyres keep in mind that on SW20's Toyota originally fitted 195/60R14 front and 205/60R14 rear
 
On 92+ cars they fitted 195/55R15 front and 225/50R15 rear, and so the story goes the wider rear was fitted because of their so called bad oversteer (in the hands of novices). 
 
Original rims also used a one inch stagger with the rim width F/R