Can we connect 48 volt batteries in series with the 12 Volt batteries
Yes, it will work, although it is not recommended to connect different output batteries in series, cause it will cause damage during the discharge and, recharging cycle. If you connect them in series, ensure the 12volts has the same Ampere hour rating. The effective voltage will be added up ie 48m+12n, m numbers of 48volts batteries, and n numbers of 12volts batteries.
You can definitely connect batteries that have different voltages in series. It is recommended to have batteries with equal current output, but it will not damage your circuit if they are different. In series circuits the output voltage will be the sum of all voltages outputs of the batteries connected in series