Is this a right way of regulating the voltage , 7805 data sheets do not specify any minimal voltage input
a quick look at a National semi datasheet show:
InputMin Voltage (Volt) 7.20


When I powered the micro and the peripheral directly without the regulator they work just fine . Is this safe to do it as 89c51RD2xx typically specify + 5.5 V max. I have seen that a new cell is 1.5V but it falls rapidly to 1.2~1.3 V and stabilises there.
No, some chips may survive, some may not.

270 mA while scanning. So it looks like this would not work out, even though 7805 is set for 1.5 Amp current.
It is not the 7805, it is the battery that sags, 270mA for a 9V flat is too much.

The skinny:
Use a switching buck/boost converter, that, with your load is, as far as I can see, the only solution unless you want to use a much larger battery such as e.g. 5 or 6 'D' cells.

Erik