with arrays it is possible, but avoid it if possible and use one EA for one Pair...
... for few reasons, in MT4 opening multipairs can not be tested in tester, only in MT5 ... and only blocks in onTick section .... on Timer section is not working in tester
... in MT4 you can test reading data from other pairs, but not opening multipair trades
if it is not possible to avoid, than create it
... if you create it in onTick section, all your pairs depends on ticks from pair (from chart) where EA is placed (good or bad? it depends on your strategy)
... if possible, create and test your strategy in forward demo with blocks placed in onTimer section ... set timer interval for some reasonable value, like 10s (here again, it depends on your strategy)
... in overall, if you create multipair EA, and your strategy is lossable in lets say 5 pairs, in other pairs is most probably lossable too, ... when you sum that, you end up logically in the loss 
... my hint is, focus for only few pairs (filter them too, because some pairs are more trendy (gbpjpy) and other pairs are more side (eurchf)
... create profitable EA for one pair (you can read data from other pairs) but with EA open only 1 pair based on that data ... read these data with multicurrency custom indicator if possible, if not possible you can do it in EA ...
working with multipair (or multicurrency) EAs is possible, be aware because it is big challenge and you can easily fail, your EA can fail and when you ended up with 28*x opened trades, you will easily lose yourself and probably a lot of money too (because closing 28 trades may not be cheap
) ...