Will softwareSerila, interfere with HIGH frequency ext interupt??

hello forum. i am writing a sketch, in which, there will be an external interupt, almost ever 0.4ms.
the ISR function, will not be heavy. just some “if” so it want take long to execute.
It is VERY important, that i do not miss any of those interupts…

i also, want to use, SoftwareSerial library…

so my question is this: I know that the SoftwareSerial uses the PinChange interupts of Atmega. and that, when an ISR is being executed, all new interupts, are being ignored…
so if the external interupt happens every 0.4ms,and i am also receiving SoftSerial, but at low speeds (a 3 digit number and a comma, every 100ms or something). any change that the 2 interupts will happen simulatinosly?? will this produce a problem?? what do you think???

(also, what is the lowest baud i can set, so i can minimize the chanches ? or should i go with the highest possible? )