Is there any way of removing certain enemies from the game?

Both is possbile.
for example you´ve got the reaper. 
You can change the reaper to a rat including behavior as a rat.
but you can also change the reaper to a rat and let behavior as the reaper, so it can me mixed.

You need Fluffy´s arc tool.
thanks to this program you can extract file in .xml.
Those .xml files are readeable ˇtext files/scriptsˇand you can change lots of things.
Some of them are too easy, some of ´em are really difficult.

after you download the arc tool (it´s here in this forum) you better dowload batch files.
then paste those .bat files to the folder whe the arc tool is.

then for Resident evil 5 there will be pc-re5.bat
just rename it to pc-re5.txt and open.
settings should be like this:  arctool -alwayscomp -re5 -texRE5 -tex -xfs -v 7 -pc %1 %2 %3 %4 %5 %6 %7 %8 %9
better copy this and paste it again.
then renamed pc-re5.txt back to pc-re5.bat.
Then everything is ready.

You need to find the stages where those reapers are.
S307 and then S314
There are reapers in these 2 stages.

If you don´t know where to find stage, then you have to open your game folder of Re5.
NativePC_MT (if you have golden edition), open that. Then find image folder and open. Then is Archive folder and there are .arc files.
S307.arc
S314.arc
These two files, copy them and paste them to Arc tool folder. Then move S307.arc to the  pc-re5.bat. It´ll extract it.

then open the extracted stage folder.
there will be three sub-folders (sometimes more)

effect
sound
stage

so again, open the stage folder.
then S307 folder.
and finally the last folder called soft.

Can you see these files?

s307_em00.lot.xml
s307_em01.lot.xml
or
s307_em02.lot.xml

These are for the enemies.

for example:
rename s307_em00.lot.xml to s307_em00.lot.txt
open it.

<?xml version="1.0" encoding="utf-8"?>

<class name="XFS" type="355479284">

<u32 name="mQuality" value="2"/>

<classref name="mSetInfos" type="1637199632">

<s32 name="mID" value="0"/>

<string name="mInfoClass" value="cSetInfoEm34"/>

<string name="mUnitClass" value="uEm34"/>

Watch value="cSetInfoEm34"/> and  value="uEm34"/>

Em34 is value of enemy reaper.
Em6c is for a rat.
So what you have to do? Just rename value="cSetInfoEm34"/> to value="cSetInfoEm6c"/> and value="uEm34"/> to value="uEm6c"/>

This is how it will look:

<?xml version="1.0" encoding="utf-8"?>

<class name="XFS" type="355479284">

<u32 name="mQuality" value="2"/>

<classref name="mSetInfos" type="1637199632">

<s32 name="mID" value="0"/>

<string name="mInfoClass" value="cSetInfoEm6c"/>

<string name="mUnitClass" value="uEm6c"/>


then rename s307_em00.lot.txt back to s307_em00.lot.xml
and then move the extracted stage folder to  pc-re5.bat again.
That will repack S307 back to S307.arc
The same you will do for the S314.arc. Same process. 
sometime you need to research, because there are stages with 20 or more enemies.
So remember, if you find the value = Em34, then that´s the right one.
It doesn´t have to be
s307_em00.lot.xml

s307_em01.lot.xml

or

s307_em02.lot.xml
It also can be s307_em04.lot.xml or s307_em15.lot.xml
Just research it.
after all you need put S307.arc back into to Archive folder, where are those .arc files.
And That´s how the reaper becomes a rat.