It is not illegal to use tyres of different load ratings on the same vehicle as long as they are within the minimum threshold specified by the vehicle manufacturer. In terms of safety, it is more important that the tyres on the same axle have the same or similar tread patterns and are of a similar specification - for example, if you have three economy-biased tyres you should ensure that the fourth tyre is similar.

The difference in load rating between a 91 and a 95-rated tyre is 75kg, so unless you are loading your vehicle to its maximum capacity we would not expect this to make a difference. In addition, it is unlikely that the difference in tyre specification is causing issues with the tyre pressure monitor, unless of course it is this particular tyre that is consistently producing warnings.